1. The problem statement, all variables and given/known data A spear moves past you at a very high speed. As it passes, you measure its length at one half its normal length. From this measurement, what can you conclude about the moving spear's mass? 2. Relevant equations 3. The attempt at a solution To have the spear appear that it is 1/2 its length means it is moving at a substantial fraction of the speed of light. And as the spear's velocity approaches the speed of light the relativistic mass appears greater and greater.... But is there some quantitative value I should know? Such as... the mass of the spear is twice its normal mass? Or some equation that relates the relativistic mass to length?